Printing Device that Controls Discharge Roller Based on Thickness of Recording Medium

ABSTRACT

A printing device includes a printing unit, a discharge roller, and a roller controller. The printing unit performs printing on a recording medium at a printing position. The discharge roller is configured to rotatably contact the recording medium on which the printing unit has performed printing, thereby discharging the recording medium. The roller controller controls the discharge roller to rotate such that the discharge roller discharges a recording medium having thickness greater than or equal to a predetermined threshold value at a first speed and that the discharge roller discharges a recording medium having thickness less than the predetermined threshold value at a second speed higher than the first speed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Japanese Patent Application No. 2006-096625 filed Mar. 31, 2006. The entire contents of the priority application are incorporated herein by reference.

TECHNICAL FIELD

The invention relates to a printing device, and more particularly to a printing device having a discharge roller for discharging a printed recording medium.

BACKGROUND

Printing devices for performing printing on recording paper (recording medium) have been widely used. In such printing devices, the recorded paper is discharged by a discharge roller and stacked on, for example, a discharge tray. When the speed of discharging the recording paper (discharge speed) is increased, the recording paper bursts onto the discharge tray at a high speed. Hence, there is a possibility that the recording paper is not neatly stacked on the discharge tray. Japanese Publication of Patent Application No. 2000-198600 discloses improving stack performance of the recording paper by decreasing the discharge speed.

SUMMARY

However, if the discharge speed is simply decreased, the following problem will occur. That is, when the thickness of the recording paper is small or the size of the recording paper is large, the trailing edge of the recording paper is not separated from the discharge roller and thus the recording paper may not be discharged.

More specifically, when a recording paper is thin and flexible (small flexural rigidity), the leading edge of the recording paper passing through the discharge roller hangs down and contacts the recording paper stacked on a discharge tray. As a result, friction occurs between the recording paper in the discharge tray and the newly discharged recording paper. Since the recording paper needs to be discharged against the frictional force, the thin and flexible recording paper requires a larger amount of energy for discharging, in comparison with a thick or less flexible recording paper which generates less friction.

Further, when the recording paper is fed from a paper cassette (not shown) to a conveying roller, the fed recording paper and the recording paper in the paper cassette rub against each other, so that the recording paper takes on static electricity. As the size of the recording paper is larger, the recording paper becomes charged with static electricity more easily. A larger amount of energy is required to discharge the recording paper against the static electricity. Due to such frictional force and static electricity, the large recording paper or the thin recording paper is hard to be discharged. When the recording paper is not discharged onto the discharge tray, the user has to remove a cover of the printing device and pull out the printed recording paper by himself or herself, which is bothersome.

To reliably discharge the recording paper against the above-mentioned frictional force and static electricity, the discharge speed should be increased. However, if the discharge speed of the recording paper is uniformly increased, the stack performance of the recording paper deteriorates. Especially when the thickness of the recording paper is large or the size of the recording paper is small, the above-mentioned frictional force and static electricity have less effect on the recording paper. Thus, the recording paper is discharged onto the discharge tray with great force, leading to poor stack performance.

In view of the foregoing, it is an object of the invention to provide a printing device capable of reliably discharging a recording medium while improving stack performance of the recording medium.

In order to attain the above and other objects, the invention provides a printing device. The printing device includes a printing unit, a discharge roller, and a roller controller. The printing unit performs printing on a recording medium at a printing position. The discharge roller is configured to rotatably contact the recording medium on which the printing unit has performed printing, thereby discharging the recording medium. The roller controller controls the discharge roller to rotate such that the discharge roller discharges a recording medium having thickness greater than or equal to a predetermined threshold value at a first speed and that the discharge roller discharges a recording medium having thickness less than the predetermined threshold value at a second speed higher than the first speed.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative aspects in accordance with the invention will be described in detail with reference to the following figures wherein:

FIG. 1 is a perspective view of a printing device according to an embodiment of the invention;

FIG. 2 is a schematic side view showing an internal configuration of the printing device;

FIG. 3 is a block diagram showing an electric configuration of the printing device;

FIG. 4 is a flow chart showing a printing process according to the embodiment;

FIG. 5 is an explanatory diagram showing a testing method for measuring paper discharge positions;

FIG. 6A is a table showing averages and standard deviations which were calculated in a first test;

FIG. 6B is a graph in which a thin line connects each point of calculated standard deviations and a thick curve is a polynomial approximation showing relationship between discharge speeds and the standard deviations, where the standard deviations are shown in a vertical axis and the discharge speeds are shown in a horizontal axis;

FIG. 6C is a graph in which a thin line connects each point of calculated averages and a thick curve is a polynomial approximation showing relationship between discharge speeds and the averages, where the averages are shown in a vertical axis and the discharge speeds are shown in a horizontal axis;

FIG. 7A is a table showing averages and standard deviations which were calculated in a second test;

FIG. 7B is a graph in which a thin line connects each point of calculated standard deviations and a thick curve is a polynomial approximation showing relationship between discharge speeds and the standard deviations, where the standard deviations are shown in a vertical axis and the discharge speeds are shown in a horizontal axis;

FIG. 7C is a graph in which a thin line connects each point of calculated averages and a thick curve is a polynomial approximation showing relationship between discharge speeds and the averages, where the averages are shown in a vertical axis and the discharge speeds are shown in a horizontal axis; and

FIG. 8 is an explanatory diagram showing a paper discharge mechanism according to a comparative example.

DETAILED DESCRIPTION

A printing device according to an embodiment of the invention will be described while referring to FIGS. 1 through 7C.

FIG. 1 is a perspective view showing a printing device 1 according to the embodiment. The printing device 1 (multifunction device) shown in FIG. 1 has a facsimile function, printer function, copier function, scanner function, and the like. The printing device 1 is configured of a main case 2 that is substantially box-shaped and open on the top, and an upper case 3 disposed on top of the main case 2 and capable of swinging open and closed about the swing axis of hinges or the like (not shown) provided on one side of the main case 2. In the following description, the left-to-right direction (main scanning direction and Y direction), front-to-rear direction (subscanning direction and X direction), and vertical direction will be based on the orientation of the printing device 1 shown in FIG. 1. The main case 2 and upper case 3 are formed of a synthetic resin according to an injection molding process.

A control panel 30 is provided on the top surface of the upper case 3 and the front end thereof. The control panel 30 includes various buttons, including numerical buttons, a Start button, and functional buttons. The user manipulates these buttons to perform various operations. The control panel 30 also includes a liquid crystal display (LCD) 31 for displaying the settings and status of the printing device 1 and various operational messages as needed.

A scanner 33 is disposed in the upper case 3 to the rear of the control panel 30. The scanner 33 has a flatbed scanning unit with a large glass plate on which an original to be scanned is placed, and a cover 34 capable of rotating over the flatbed scanning unit to cover or expose the top surface thereof. The scanner 33 functions to read images from a facsimile original to be transferred to another facsimile device with the facsimile function or an original to be copied with the copy function.

While not shown in the drawings, a line-type contact image sensor is disposed beneath the glass plate of the flatbed scanning unit. The contact image sensor is one example of a photoelectric conversion device for scanning the image surface of an original placed on the glass plate. The cover 34 is capable of swinging open and closed about hinges provided on the rear side of the printing device 1 (far side in FIG. 1).

As shown in FIG. 1, a paper cassette 5 is provided in the left-to-right center of the main case 2. The paper cassette 5 can accommodate a plurality of sheets of a recording paper P in a stacked and substantially level state on the bottom surface thereof. The paper cassette 5 can be pulled out through an opening 2 a formed in the front surface of the main case 2. A discharge tray 10 is provided above the paper cassette 5. The printed recording paper P is discharged onto the discharge tray 10 in a direction indicated by an arrow B, with its printed surface oriented upward. The opening 2 a is also in communication with the discharge tray 10.

FIG. 2 is a side view conceptually illustrating the internal structure of the printing device 1. In FIGS. 1 and 2, an arrow B indicates the conveying (feeding) direction of the recording paper P. As shown in FIG. 2, a feeding unit 6 is accommodated in the main case 2. The feeding unit 6 includes a feeding roller 7 disposed above the paper cassette 5. The feeding unit 6 is configured to feed sheets of the recording paper P along a conveying path formed in a U-shape in the rear end of the main case 2 so that the sheet is conveyed upward, inverted, and conveyed back in the forward direction (left in FIG. 2).

Positioned downstream of the U-shaped section of the conveying path with respect to the conveying direction of the recording paper P (B direction) are provided a conveying roller 20 a for conveying the recording paper P, and a pinch roller 20 b for pressing the recording paper against the conveying roller 20 a. Working together, the conveying roller 20 a and pinch roller 20 b pinch convey the sheets of recording paper P. A flat platen 11 is disposed downstream of the conveying roller 20 a and pinch roller 20 b for supporting the sheets of recording paper P conveyed by the conveying roller 20 a and pinch roller 20 b.

Further downstream of the platen 11 in the conveying direction of the recording paper P are provided a discharge roller 21 a for receiving sheets of recording paper P conveyed from the conveying roller 20 a and for discharging the sheets from the main case 2, and a pinch roller 21 b for pressing the recording paper P against the discharge roller 21 a. The discharge tray 10 is provided downstream of the discharge roller 21 a and pinch roller 21 b for discharging the printed recording paper P. Further, a recording head 12 is positioned above the platen 11 between the conveying roller 20 a and discharge roller 21 a and above the conveying path along which the recording paper P is conveyed.

The recording head 12 is mounted in a carriage 13 that can reciprocate in the main scanning direction (direction orthogonal to the plane of the drawing in FIG. 2). Nozzles (not shown) are formed in the recording head 12 on the recording paper P side for ejecting ink toward the recording paper P pinched by the conveying roller 20 a and the like. The position on the platen 11 that confronts the nozzles of the recording head 12 serves as a printing position 14.

Ink tanks for supplying ink to the recording head 12 are detachably mounted in an accommodating section (not shown) of the main case 2 from an upper side. In the present embodiment, the recording head 12 is configured to perform color printing, and the accommodating section accommodates ink tanks in the four colors black, cyan, magenta, and yellow. However, the accommodating section may be configured to accommodate ink tanks for a greater or smaller number of colors. Flexible ink tubes connect the ink tanks to the recording head 12 for supplying ink thereto.

In the present embodiment, a linefeed motor 42 configured of a DC motor produces a rotational force for rotating the conveying roller 20 a. The rotational force of the linefeed motor 42 is also transmitted to the discharge roller 21 a and the feeding roller 7 via a drive gear 101 that rotates together with the conveying roller 20 a. The mechanism for transmitting the drive force from the conveying roller 20 a to the discharge roller 21 a is well known, and is not shown in the drawings and will not be described herein. The conveying roller 20 a and the discharge roller 21 a are driven to rotate in synchronicity with each other.

As shown in FIG. 2, the driving force of the conveying roller 20 a is transmitted via the drive gear 101 to a gear train 43 configured of a plurality of mating gears 43 a, 43 b, 43 c, and 43 d. The drive force is in turn transmitted via the gear train 43 to a drive shaft 14 of the feeding unit 6. Next, the driving force of the drive shaft 14 is transmitted to the feeding roller 7 via a gear train 50 configured of a plurality of mating gears.

In the present embodiment, rotation of the linefeed motor 42 in a direction driving the conveying roller 20 a and the discharge roller 21 a to convey the recording paper P in the conveying direction B is called a forward rotation, and rotation of the linefeed motor 42 in a direction driving the conveying roller 20 a and the discharge roller 21 a to convey the recording paper P in the direction opposite the conveying direction B (the direction for returning the recording paper P toward the paper cassette 5) is called a reverse rotation. In FIG. 2 arrows formed of solid lines (arrows F) indicate the rotational direction of each roller and gear during a forward rotation of the linefeed motor 42, while arrows formed of dotted lines (arrows R) indicate the rotational direction during a reverse rotation of the linefeed motor 42.

When the linefeed motor 42 rotates in reverse, the conveying roller 20 a rotates counterclockwise in FIG. 2. Accordingly, the feeding roller 7 is driven to rotate in the feeding direction (counterclockwise in FIG. 2) through the gear train 50 of the feeding unit 6. In this way, the feeding roller 7 begins to feed the topmost sheet of the recording paper P stacked in the paper cassette 5. At this time, the conveying roller 20 a rotates in the direction opposite from the direction for conveying the recording paper P in the conveying direction B. Thus, even when the recording paper P is slanted, the leading edge of the recording paper P is temporarily stopped at the conveying roller 20 a to correct the slant.

After the sheet of recording paper P fed from the paper cassette 5 reaches the conveying roller 20 a, the linefeed motor 42 rotates forward a prescribed amount. This is a cueing operation in which the recording paper P becomes pinched between the conveying roller 20 a and pinch roller 20 b and is conveyed below the recording head 12. The cueing operation is an operation for setting the recording paper P in a position where printing can begin by advancing the leading edge of the recording paper P to a prescribed position over the platen 11. During the cueing operation, the feeding roller 7 rotates in the direction opposite the feeding direction (clockwise in FIG. 2). However, since the nip force between the conveying roller 20 a and pinch roller 20 b is set greater than the conveying force of the feeding roller 7, the feeding roller 7 does not return the recording paper P to the paper cassette 5, but rather the conveying roller 20 a and pinch roller 20 b gripping the leading edge of the recording paper P convey the recording paper P forward as the recording paper P slips over the outer surface of the feeding roller 7.

Next, the conveying roller 20 a begins advancing the recording paper P intermittently, while the recording head 12 mounted on the carriage 13 begins printing by ejecting ink through the nozzles onto one surface of the recording paper P as the carriage 13 reciprocates in the main scanning direction. As shown in FIG. 2, both the conveying roller 20 a and the discharge roller 21 a rotate in the conveying direction of the recording paper P when advancing the recording paper P intermittently.

After printing is complete for one sheet of the recording paper P, the printing device 1 begins to discharge the printed sheet of recording paper P. At this time, the linefeed motor 42 rotates forward a prescribed amount sufficiently for continuously rotating the conveying roller 20 a and discharge roller 21 a in the discharging direction, after which the linefeed motor 42 is halted.

A rotary encoder 44 is also provided on one end of the conveying roller 20 a. The rotary encoder 44 is configured of a slit plate 44 a having slits (not shown) formed at prescribed intervals along the circumferential direction of the plate, and an optical sensor 44 b. The optical sensor 44 b converts light passing through the slits in the slit plate 44 a to an electric signal and outputs the signal. Hence, the slits in the slit plate 44 a can be detected based on the electric signal outputted from the optical sensor 44 b.

The slit plate 44 a rotates coaxially with the conveying roller 20 a. As described above, the linefeed motor 42 rotates the conveying roller 20 a. Hence, by counting the number of slits detected by the rotary encoder 44, it is possible to calculate not only the rotational amount of the conveying roller 20 a, but also the rotational amounts of the linefeed motor 42.

A paper sensor 117 is also disposed upstream of the conveying roller 20 a in the conveying direction B. The paper sensor 117 detects the leading and trailing edges of the recording paper P when the recording paper P passes through the U-shaped section of the conveying path and approaches the printing position 14. The position of the recording paper P along the conveying path can be determined based on detections by the paper sensor 117.

FIG. 3 is a block diagram showing the primary electrical structure of the printing device 1 described above. As shown in FIG. 3, the printing device 1 includes a CPU 300, a ROM 301, a RAM 302, and a EEPROM 303 that are all connected to an ASIC (application specific integrated circuit) 306 via a bus 305. The printing device 1 also includes the paper sensor 117, the rotary encoder 44, a linear encoder 37, the recording head 12, a drive circuit 314 for driving the recording head 12, a carriage motor 24, a drive circuit 312 for driving the carriage motor 24, the linefeed motor 42, a drive circuit 311 for driving the linefeed motor 42, a keyboard 30 a, the LCD 31, a panel interface 313 connected to the keyboard 30 a and the LCD 31, a parallel interface 315, a USB interface 316, and a NCU (network control unit) 317.

The CPU 300 is a central processing unit that controls overall operations of the printing device 1. The CPU 300 executes various programs, including a program for implementing the processes shown in FIG. 4. The ROM 301 is a non-rewritable memory storing various control programs, including a program for implementing the operations in FIG. 4, and data required by the CPU 300 when executing the control programs.

The RAM 302 temporarily stores programs and data required for various processes executed by the CPU 300. The RAM 302 includes a paper size memory 302 a and a paper thickness memory 302 b. The paper size memory 302 a is a memory for storing the size of the recording paper P used in a printing process described later (FIG. 4). The paper thickness memory 302 b is a memory for storing the thickness of the recording paper P used in the printing process.

The ASIC 306 is also connected to the NCU 317 having a modem 318. A communication signal received by the NCU 317 from a public network is first demodulated by the modem 318 and then inputted into the ASIC 306. Further, when the ASIC 306 transmits image data externally for a facsimile transmission or the like, the modem 318 modulates the image data to produce a communication signal and outputs this signal to the public network via the NCU 317.

In addition, the ASIC 306 generates a phase excitation signal or the like for energizing the linefeed motor 42, for example, based on commands from the CPU 300 and applies this signal to the drive circuit 311 of the linefeed motor 42 and the drive circuit 312 of the carriage motor 24, thereby supplying a drive signal to the linefeed motor 42 and carriage motor 24 via the drive circuit 311 and drive circuit 312 for controlling the forward and reverse rotations of the linefeed motor 42 and carriage motor 24, halting the motors, and the like. The ASIC 306 is also connected to the drive circuit 314 that drives the recording head 12 to selectively eject ink onto the recording paper P at a prescribed timing. The ASIC 306 generates and outputs a signal based on a drive control procedure received from the CPU 300, and the drive circuit 314 controls the recording head 12 based on the signal outputted from the ASIC 306.

The ASIC 306 is also connected to the panel interface 313 for facilitating transmission and reception operations with the keyboard 30 a and the LCD 31 of the control panel 30, and the parallel interface 315 and USB interface 316 for exchanging data with a personal computer or other external device via a parallel cable or USB cable, respectively.

Further, the ASIC 306 is connected to the paper sensor 117 positioned upstream of the conveying roller 20 a in the conveying direction for detecting the leading and trailing edges of the recording paper P as the recording paper P is fed toward the printing position 14 (FIG. 2), the rotary encoder 44 for detecting rotational amounts of the conveying roller 20 a and the feeding roller 7, and the linear encoder 37 for detecting the amount that the carriage 13 moves in the main scanning direction and the current position of the carriage 13.

Next, processes executed on the printing device 1 of the present embodiment will be described with reference to the flowchart in FIG. 4. FIG. 4 is a flowchart illustrating steps in a printing process initiated when a print command is issued from an external device or the like (not shown).

In S2 the CPU 300 acquires the paper size selected by the user and stores the paper size in the paper size memory 302 a. In S4 the CPU 300 acquires the paper thickness selected by the user and stores the paper thickness in the paper thickness memory 302 b. For example, the user selects the paper size and the paper thickness on a printing setting screen of a personal computer. When the user gives a printing instruction from the personal computer, the paper size and the paper thickness are included in printing data. Thus, the CPU 300 stores the paper size and the paper thickness in the paper size memory 302 a and the paper thickness memory 302 b, respectively.

In S6 the CPU 300 controls the linefeed motor 42 to rotate reverse to rotate the sheet feeding roller 7 to separate only an uppermost recording paper P from a plurality of sheets of the recording paper P stacked in the paper cassette 5 and to feed the uppermost recording paper P until the leading edge of the recording paper P reaches the conveying roller 20 a.

In S8 the CPU 300 controls the linefeed motor 42 to rotate forward by a predetermined amount to rotate the conveying roller 20 a to convey the recording paper P nipped at a nip part between the pinch roller 20 b and the conveying roller 20 a to the printing position 14.

In S10 the CPU 300 controls the carriage motor 24 and the recording head 12 to perform printing for one line on the recording paper P while the carriage 13 is reciprocated. In S12 the CPU 300 controls the linefeed motor 42 to rotate forward by a predetermined amount to rotate the conveying roller 20 a for conveying the recording paper P by one linefeed in the conveying direction B. In S14 the CPU 300 determines whether printing for one page is completed. When printing for one page is not completed (S14: No), the CPU 300 drives the recording head 12 and the carriage 13 again to repeat printing for one linefeed and conveying for one linefeed (S10, S12).

When printing for one page is completed in this manner (S14: Yes), in S16 the CPU 300 controls the linefeed motor 42 to rotate forward by a predetermined amount to rotate the conveying roller 20 a and the discharge roller 21 a, thereby conveying the recording paper P at a discharge speed of 20 inches/second (ips) in the conveying direction B until the trailing edge of the printed recording paper P reaches a position which is 15 mm (millimeters) upstream from the discharge roller 21 a (more specifically, a position which is 15 mm upstream from a nip position between the discharge roller 21 a and the pinch roller 21 b in the conveying direction B).

In S18, referring to the paper thickness memory 302 b, the CPU 300 determines whether the thickness of the recording paper P is greater than or equal to a predetermined threshold (0.21 mm in the present embodiment). When the thickness of the recording paper P is greater than or equal to 0.21 mm, that is, the recording paper P is thick (S18: Yes), in S22 the CPU 300 controls the linefeed motor 42 to rotate forward by a predetermined amount so that the discharge roller 21 a discharges the recording paper P onto the discharge tray 10 at a discharge speed of 4 inches/second.

On the other hands when the thickness of the recording paper P is less than 0.21 mm (S18: No), referring to the paper size memory 302 a, in S20 the CPU 300 compares the size of the recording paper P with DIN A4 size. In other words, the CPU 300 determines whether the paper size stored in the paper size memory 302 a is less than A4 size. When the size of the recording paper P is less than A4 size (S20: Yes), in S22 the CPU 300 controls the linefeed motor 42 to rotate forward and controls the discharge roller 21 a to rotate so that the recording paper P is discharged onto the discharge tray 10 at a discharge speed of 4 inches/second.

On the other hand, when the thickness of the recording paper P is less than 0.21 mm (SIB: No) and the paper size is A4 size (S20: No), in S24 the CPU 300 controls the linefeed motor 42 to rotate forward and controls the discharge roller 21 a to rotate so that the recording paper P is discharged onto the discharge tray 10 at a discharge speed of 13 inches/second. The thin or large recording paper P is hard to be separated from the discharge roller 21 a due to frictional force and static electricity. However, the recording paper P can be reliably separated by discharging the recording paper P at such a high speed.

In S26 the CPU 300 determines whether data exists for the recording paper P of next page. When data exists (S26: Yes), the process returns to S6 and feeding of the next recording paper P is started. In this case, since the linefeed motor 42 is rotated reverse, the discharge roller 21 a is also rotated in the direction opposite the direction in which the recording paper P is discharged. However, since the recording paper P is reliably separated from the discharge roller 21 a and discharged, it can be prevented that the recording paper P caught by the discharge roller 21 a is fed back. When all data is printed by repeating the above-mentioned process (S26: No), the process ends.

According to the printing process in the present embodiment, rotation of the discharge roller 21 a is controlled so that the thin and large recording paper P of A4 size having a thickness less than 0.21 mm is discharged at a higher speed than the thick recording paper P having a thickness greater than or equal to 0.21 mm or the recording paper P smaller than A4 size. Accordingly, even the thin or large recording paper P which is greatly affected by static electricity and frictional force can be reliably discharged. On the other hand, since the thick or small recording paper P which is less affected by static electricity and frictional force is discharged at a low speed, stack performance after discharging is improved.

Referring to FIGS. 5 through 7C, results of tests of discharging the recording paper P at different discharge speeds using a conveying mechanism and a discharge mechanism in the printing device 1 will be described.

As shown in FIG. 5, in the tests, the discharge tray 10 was provided with a stopper 10 a. One hundred (100) sheets of the recording paper P were stacked on the discharge tray 10 beforehand such that the discharged recording paper P would not hit the stopper 10.

First, a test of discharging a golden copy sheet (manufactured by Optima Corp., thickness of 0.1 mm) of A4 size was carried out as a first test. In the first test, the recording paper P was conveyed at 20 inches/second until the trailing edge of the recording paper P is at a position which is 15 mm upstream from the discharge roller 21 a. Then, the discharge speed was changed after the trailing edge of the recording paper P passed the position which is 15 mm upstream from the discharge roller 21 a, and a paper discharge position was measured. Here, a position Q1 of the stopper 10 a is referred to as a reference position O. A direction in which the recording paper P moves away from the discharge roller 21 a (the same direction as the conveying direction B) is referred to as a plus (+) direction. A direction in which the recording paper P gets closer to the discharge roller 21 a (the direction opposite the conveying direction B) is referred to as a minus (−) direction. Specifically, using the left and right end positions of the leading edge of the discharged recording paper P as measurement points, distances between the measurement points and the reference position O were measured and an average of the distances was determined as the paper discharge position of the recording paper P.

Ten (10) tests were carried out for each of different discharge speeds. An average and a standard deviation σ of the paper discharge positions obtained through the ten tests were calculated.

FIG. 6A is a table showing averages and standard deviations σ which are calculated in the first test. FIG. 6B is a graph in which a thin line connects each point of calculated standard deviations σ and a thick curve is a polynomial approximation showing relationship between discharge speeds and the standard deviations σ, where the standard deviations σ are shown in a vertical axis and the discharge speeds are shown in a horizontal axis. FIG. 6C is a graph in which a thin line connects each point of calculated averages and a thick curve is a polynomial approximation showing relationship between discharge speeds and the averages, where the averages are shown in a vertical axis and the discharge speeds are shown in a horizontal axis.

The standard deviation σ in FIG. 6B is a value representing variation in the paper discharge positions, and thus indicates stack performance of the recording paper P (whether the sheets of the recording paper P are stacked neatly). As the standard deviation σ becomes larger, variation in the paper discharge position is larger, which indicates that stack performance is worse. FIG. 6B shows that the standard deviation σ rapidly increases when the discharge speed exceeds 14 inches/second (ips). The reason is as follows: when the discharge speed is high, kinetic energy of the recording paper P during discharge is large. Thus, the recording paper P is easily affected by various factors (attitude or direction of the recording paper during discharge, static electricity, frictional force, and the like), resulting in large variation. In the first test, the stack performance is determined as OK when σ is less than 4.5 mm and as NG when σ is greater than or equal to 4.5 mm.

The average in FIG. 6C is an average of leading edge positions of the recording paper P discharged at the different discharge speeds. When the average is large, the recording paper P is discharged to a position sufficiently away from the discharge roller 21 a. When the average is small, the recording paper P is discharged to a position in vicinity of the discharge roller 21 a. Especially the thin (0.1 mm) A4 golden copy sheet used in the first test is easily affected by static electricity and frictional force and cannot be easily separated from the discharge roller 21 a. Thus, when the discharge speed is low, there are cases where the recording paper P cannot be separated from the discharge roller 21 a. If the linefeed motor 42 is rotated reverse to feed the next recording paper P in the state where the recording paper P is not separated from the discharge roller 21 a, the recording paper P caught by the discharge roller 21 a is fed back, thereby leading to a paper jam.

The longitudinal length of the A4 recording paper P used in the first test is 297 mm. A distance L between a position Q2 of the discharge roller 21 a and the position Q1 of the stopper 10 a was set to 322 mm which was obtained by adding 25 mm to the length of the A4 recording paper P. Thus, when the leading edge of the discharged recording paper P is located upstream (at the discharge roller 21 a side) than −25 mm position, the recording paper P is caught by the discharge roller 21 a and fed back. Further, as described above, the discharge positions of the recording papers P vary around the average. Hence, in the first test, in consideration of the variation of 20 mm, if the average of the paper discharge positions is greater than or equal to −5 mm (that is, the trailing edge of the recording paper P is separated from the discharge roller 21 a by greater than or equal to 20 mm on the average), it is determined as “no backward feed (backward feed OK)”. On the other hand, if the average of the paper discharge positions is less than −5 mm, it is determined as “possible backward feed (backward feed NG)”.

As described above, determination results on stack performance and backward feed for each discharge speed are summarized in FIG. 6A. As shown in FIG. 6A, the discharge speed from 12 inches/second to 14 inches/second is suitable for the A4 golden copy sheet. As shown in FIG. 6A, since the discharge speed greater than or equal to 12 inches/second results in the average of the paper discharge positions greater than or equal to −5 mm, the recording paper P can be reliably separated from the discharge roller 21 a and discharged irrespective of static electricity and frictional force. In addition, by setting the discharge speed to less than 14 inches/second, stack performance of the recording paper P can be prevented from being impaired.

Next, a test of discharging an L size sheet (89 mm×127 mm, thickness of 0.21 mm) was carried out as a second test. In the second test, the recording paper P was conveyed at 20 inches/second until the trailing edge of the recording paper P is at a position which is 15 mm upstream from the discharge roller 21 a. Then, after the trailing edge of the recording paper P passed the position which is 15 mm upstream from the discharge roller 21 a, the discharge speed was changed and a paper discharge position was measured. Here, the position Q2 of the discharge roller 21 a is referred to as a reference position O (refer to FIG. 5). A direction in which the recording paper P moves away from the discharge roller 21 a (the same direction as the conveying direction B) is referred to as a plus (+) direction. A direction in which the recording paper P gets closer to the discharge roller 21 a (the direction opposite the conveying direction B) is referred to as a minus (−) direction. Specifically, the left and right end positions of the trailing edge of the discharged recording paper P from the reference position O in the conveying direction B were measured, and an average of the measured positions was determined as the paper discharge position of the recording paper P.

Ten (10) tests were carried out for each discharge speed of different discharge speeds. An average and a standard deviation σ of the paper discharge positions obtained through the 10 tests were calculated.

FIG. 7A is a table showing averages and standard deviations σ which are calculated in the second test. FIG. 7B is a graph in which a thin line connects each point of calculated standard deviations σ and a thick curve is a polynomial approximation showing relationship between discharge speeds and the standard deviations σ, where the standard deviations σ are shown in a vertical axis and the discharge speeds are shown in a horizontal axis. FIG. 7C is a graph in which a thin line connects each point of calculated averages and a thick curve is a polynomial approximation showing relationship between discharge speeds and the averages, where the averages are shown in a vertical axis and the discharge speeds are shown in a horizontal axis.

FIG. 7B shows that the standard deviation σ rapidly increases when the discharge speed exceeds 6 inches/second. In the second test, the stack performance is determined as OK when σ is less than 4.5 mm and as NG when σ is greater than or equal to 4.5 mm.

The average in FIG. 7C is an average of trailing edge positions of the recording paper P discharged at each discharge speed. As in the first test, in consideration of the variation, if the trailing edge of the recording paper P is separated from the position Q2 by greater than or equal to 20 mm on the average, it is determined as “no backward feed (backward feed OK)”. On the other hand, if the trailing edge of the recording paper 2 is separated from the position Q2 by less than 20 mm on the average, it is determined as “possible backward feed (backward feed NG)”.

As a result of the second test, it was revealed that the discharge speed from 4 inches/second to 6 inches/second is suitable for the recording paper P of L-size. By setting the discharge speed to greater than or equal to 4 inches/second, the printing rate can be prevented from decreasing. In addition, by setting the discharge speed to 6 inches/second or less, stack performance can be improved.

While the invention has been described in detail with reference to the above aspects thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention.

For example, in the above-described embodiment, size and thickness are acquired based on user settings. However, the method for acquiring size and thickness of the recording paper P is not limited to this. For example, the printing device 1 may be configured so that a sensor is provided on the carriage 13, size and thickness of the recording paper P is detected by the sensor prior to printing by the recording head 12, and the discharge speed is determined based on the detection results.

In the above-described embodiment, the discharge speed is determined based on size and thickness of the recording paper P. However, the discharge speed may be determined based on only the thickness.

In the above-described embodiment, using 0.21 mm as a single threshold of thickness, the discharge speed is determined from two kinds of discharge speeds based on whether the thickness is greater than or equal to the threshold, or less than the threshold. However, a plurality of thresholds may be set so as to determine the discharge speed in greater number of segments.

In the above-described embodiment, using A4 size as a standard size, the recording paper smaller than A4 size is discharged at the discharge speed from 4 inches/second to 6 inches/second. However, the discharge speed may be changed using letter size as a reference size, for example. Although description of detailed test results is omitted, even when the recording paper P of letter size having a thickness less than 0.21 mm is used, the same effects as in the above-described A4 recording paper P having a thickness less than 0.21 mm can be obtained by setting the discharge speed from 12 inches/second to 14 inches/second.

The printing device 1 may be configured such that the recording paper P larger than A4 size is discharged at a higher discharge speed than that of the recording paper P in A4 size.

COMPARATIVE EXAMPLE

A printing device according to a comparative example will be described with reference to FIG. 8. In FIG. 8, a recording head 112 mounted on a carriage 113 ejects ink on a recording paper P on a platen 111 while reciprocating in a main scanning direction (the direction perpendicular to the sheet of FIG. 8) to perform printing. A position between the recording head 112 and the platen 111 is referred to as a printing position 114. As shown in FIG. 8, the recording paper P held between a conveying roller 120 a and a pressing roller 120 b opposed to the conveying roller 120 a is fed to the printing position 114 by rotation of the conveying roller 120 a and reaches a discharge roller 121 a. Here, the recording paper P is held between the discharge roller 121 a and a pressing roller 121 b opposed to the discharge roller 121 a and discharged to a discharge tray 122 by rotation of the discharge roller 121 a.

When the recording paper P is thin and flexible (small flexural rigidity), the leading edge of the recording paper P passing through the discharge roller 11 a hangs down and contacts the recording paper stacked in the discharge tray 122. As a result, friction occurs between the recording paper in the discharge tray 122 and the newly discharged recording paper P. Since the recording paper P needs to be discharged against the frictional force, the thin and flexible recording paper P requires a larger amount of energy for discharging, in comparison with a thick or less flexible recording paper P which generates less friction.

Further, when the recording paper P is fed from a feeding tray (not shown) to the conveying roller 120 a, the fed recording paper P and the recording paper in the feeding tray rub against each other, so that the recording paper P takes on static electricity. As the size of the recording paper P is larger, the recording paper P becomes charged with static electricity more easily. A larger amount of energy is required to discharge the recording paper P against the static electricity. Due to such frictional force and static electricity, the large recording paper P or the thin recording paper P is hard to be discharged. When the recording paper P is not discharged onto the discharge tray, the user has to remove a cover of the printing device and pull out the printed recording paper P by himself or herself, which is bothersome. However, these problems do not occur in the printing device 1 in the above-described embodiment. 

1. A printing device comprising: a printing unit that performs printing on a recording medium at a printing position; a discharge roller that is configured to rotatably contact the recording medium on which the printing unit has performed printing, thereby discharging the recording medium; and a roller controller that controls the discharge roller to rotate such that the discharge roller discharges a recording medium having thickness greater than or equal to a predetermined threshold value at a first speed and that the discharge roller discharges a recording medium having thickness less than the predetermined threshold value at a second speed higher than the first speed.
 2. The printing device according to claim 1, further comprising: a feeding cassette that is configured to accommodate at least one sheet of the recording medium; a feeding roller that feeds the recording medium from the feeding cassette; a conveying roller that is configured to contact the recording medium fed by the feeding roller and to rotate in a conveying direction for conveying the recording medium to the printing position; and a motor that rotates the discharge roller and the conveying roller in synchronicity with each other, such that the conveying roller rotates in the conveying direction when the discharge roller rotates in a direction for discharging the recording medium, wherein the roller controller controls the motor to rotate the conveying roller and the feeding roller such that the conveying roller rotates in a direction opposite the conveying direction while the feeding roller rotates in a direction for feeding the recording medium.
 3. The printing device according to claim 2, wherein the motor is configured to rotate all of the discharge roller, the conveying roller, and the feeding roller.
 4. The printing device according to claim 1, wherein the roller controller controls the discharge roller to rotate such that the discharge roller conveys the recording medium up to a predetermined position at a third speed higher than the second speed and that, from the predetermined position, the discharge roller discharges the recording medium at either the first speed or the second speed depending on the thickness of the recording medium.
 5. The printing device according to claim 4, further comprising: a thickness determining section that determines whether the thickness of the recording medium is greater than or equal to the predetermined threshold value; and a size comparing section that compares a size of the recording medium with a reference size that is either one of an A4 size and a letter size, wherein the roller controller controls the discharge roller to rotate such that the discharge roller conveys the recording medium up to the predetermined position at the third speed and that, from the predetermined position, the discharge roller discharges the recording medium at a discharge speed higher than or equal to 12 inches per second and lower than or equal to 14 inches per second, when the thickness determining section that determines that the thickness of the recording medium is less than the predetermined threshold value and the size comparing section determines that the size of the recording medium is the reference size.
 6. The printing device according to claim 5, wherein the roller controller controls the discharge roller to rotate such that the discharge roller conveys the recording medium up to the predetermined position at the third speed and that, from the predetermined position, the discharge roller discharges the recording medium at a discharge speed higher than or equal to 4 inches per second and lower than or equal to 6 inches per second, when the thickness determining section that determines that the thickness of the recording medium is greater than or equal to the predetermined threshold value or when the size comparing section determines that the size of the recording medium is smaller than the reference size.
 7. The printing device according to claim 4, wherein the predetermined position is a position at which a trailing edge of the recording medium is 15 millimeters upstream from the discharge roller in a direction in which the recording medium is discharged. 